Valve assembly with isolation valve vessel

ABSTRACT

Apparatuses for reducing or eliminating Type 1 LOCAs in a nuclear reactor vessel. A nuclear reactor including a nuclear reactor core comprising a fissile material, a pressure vessel containing the nuclear reactor core immersed in primary coolant disposed in the pressure vessel, and an isolation valve assembly including, an isolation valve vessel having a single open end with a flange, a spool piece having a first flange secured to a wall of the pressure vessel and a second flange secured to the flange of the isolation valve vessel, a fluid flow line passing through the spool piece to conduct fluid flow into or out of the first flange wherein a portion of the fluid flow line is disposed in the isolation valve vessel, and at least one valve disposed in the isolation valve vessel and operatively connected with the fluid flow line.

CLAIM OF PRIORITY

This application is a divisional of U.S. patent application Ser. No.13/864,377, filed Apr. 17, 2013, now U.S. Pat. No. 9,721,685, whichclaims the benefit of U.S. Provisional Application No. 61/625,326 filedApr. 17, 2012, the entire disclosures of which are incorporated byreference herein.

BACKGROUND

The following relates to the nuclear power reactor arts, nuclearreaction coolant system arts, nuclear power safety arts, and relatedarts.

Light water nuclear reactors are known for maritime and land based powergeneration applications and for other applications. In such reactors, anuclear reactor core comprising a fissile material (for example, ²³⁵U)is disposed in a pressure vessel and immersed in primary coolant water.The radioactive core heats the primary coolant in the pressure vessel,and the pressure vessel (or an external pressurizer connected with thepressure vessel by piping) includes suitable devices, such as heatersand spargers, for maintaining the primary coolant at a designed pressureand temperature, e.g. in a subcooled state in typical pressurized waterreactor (PWR) designs, or in a pressurized boiling water state inboiling water reactor (BWR) designs. Various vessel penetrations takeprimary coolant into and out of the pressure vessel.

For example, in some PWR designs primary coolant is passed throughlarge-diameter penetrations to and from an external steam generator togenerate steam for driving a turbine to generate electrical power.Alternatively, an integral steam generator is located inside the reactorpressure vessel, which has advantages such as compactness, reducedlikelihood of a severe loss of coolant accident (LOCA) event due to thereduced number and/or size of pressure vessel penetrations, retention ofthe radioactive primary coolant entirely within the reactor pressurevessel, and so forth. Additional smaller diameter vessel penetrationsare provided to add primary coolant (i.e., a makeup line) or removeprimary coolant (i.e., a letdown line). These lines are typicallyconnected with an external reactor coolant inventory and purificationsystem (RCIPS) that maintains a reservoir of purified primary coolant.Further vessel penetrations may be provided to connect with an externalsteam generator, an emergency condenser, or for other purposes.

Light water reactors are evaluated to determine their response in theevent that a pipe outside of the reactor vessel breaks and a loss ofcoolant accident (LOCA) occurs. The compact integral reactor design wasdeveloped, in part, to minimize the consequence of an external pipebreak by eliminating large-diameter piping leading to and from externalsteam generators. However, integral reactors still utilize small boreconnecting piping that transports reactor coolant to and from thereactor vessel. For example, in reactors with an integral pressurizerthe reactor vessel has penetrations at the top for pressurizer spray andventing. Some emergency core cooling system (ECCS) designs includepiping connecting with an emergency condenser. The vessel also hasmakeup and letdown penetrations for coolant makeup, letdown, and decayheat removal. These lines run from the vessel to one of two valve roomswhere isolation valves act to limit loss of water for breaks down streamof the valve rooms.

This arrangement results in three categories of LOCAs. Type 1 LOCAsresult from a leak between the vessel and the valve room. Type 2 LOCAsresult from a least at penetrations in the upper vessel. Type 3 LOCAsresult from leaks that occur in the valve rooms. Type 2 and Type 3 LOCAsdo not drain the reactor water storage tanks RWSTs at the end of theLOCA and result in long term cooling using the water left in the RWSTs.Type 1 LOCAs drain coolant into the refueling cavity, draining theRWSTs.

BRIEF DESCRIPTION

The present disclosure sets forth apparatuses for reducing oreliminating Type 1 LOCAs. In accordance with one aspect, a nuclearreactor comprises a nuclear reactor core comprising a fissile material,a pressure vessel containing the nuclear reactor core immersed inprimary coolant disposed in the pressure vessel, and an isolation valveassembly including, an isolation valve vessel having a single open endwith a flange, a spool piece having a first flange secured to a wall ofthe pressure vessel and a second flange secured to the flange of theisolation valve vessel, a fluid flow line passing through the spoolpiece to conduct fluid flow into or out of the first flange wherein aportion of the fluid flow line is disposed in the isolation valvevessel, and at least one valve disposed in the isolation valve vesseland operatively connected with the fluid flow line.

The spool piece and the isolation valve vessel can cooperatively definea sealed volume capable of withstanding an operating pressure of thepressure vessel of the nuclear reactor. The at least one valve can be acheck valve preventing fluid flow out of the pressure vessel. The fluidflow line can be a makeup line for supplying reactor coolant to thepressure vessel and the at least one valve is a check valve preventingprimary coolant from flowing out of the pressure vessel through thefluid flow line. The at least one valve can comprise at least two valvesarranged in series on the fluid flow line. At least one valve caninclude an actuator for moving the valve between open and closedpositions. The actuator can be an electric, hydraulic, pneumatic ormanual actuator. The fluid flow line can be a letdown line that removesreactor coolant from the pressure vessel responsive to the actuatoropening the at least one valve. An end of the fluid flow line can bedisposed coaxially inside the spool piece. A redundant valve can bedisposed outside of the isolation valve vessel and operatively connectedwith the fluid flow line.

In accordance with another aspect, an apparatus comprises an isolationvalve assembly including an isolation valve vessel, a mounting flangesealing with the isolation valve vessel to define a sealed volume, afluid flow line in fluid communication with the mounting flange to flowfluid through the mounting flange, and a valve disposed in the isolationvalve vessel inside the sealed volume and operatively connected with thefluid flow line.

The isolation valve assembly can further include a forging including themounting flange and a second flange to which the isolation valve vesselis secured, the forging having a passageway extending between themounting flange and the second flange through which the fluid flow linepasses. The valve can be a check valve allowing flow out of the mountingflange and blocking flow into the mounting flange. The valve can includefirst and second valves disposed in the isolation valve vessel insidethe sealed volume and arranged in series along the fluid flow line. Theisolation valve assembly can further include an external isolation valvedisposed outside the isolation valve vessel and outside the sealedvolume and operatively connected with the fluid flow line. The valve caninclude an actuator for moving the valve between open and closedpositions. The actuator can be an electric, hydraulic, pneumatic ormanual actuator.

The apparatus can further comprise a nuclear reactor comprising (i) apressure vessel including a mating flange and (ii) a nuclear reactorcore comprising fissile material disposed in the pressure vessel,wherein the mounting flange of the isolation valve is connected with themating flange of the pressure vessel of the nuclear reactor. The fluidflow line can be a makeup line of a reactor coolant inventory andpurification system (RCIPS) and the valve can be a check valvepreventing backflow of reactor coolant from the pressure vessel into themakeup line. The fluid flow line can be a coolant letdown line of areactor coolant inventory and purification system (RCIPS) and the valvecan be an actuated valve.

In accordance with still another aspect, a nuclear reactor comprises anuclear reactor core comprising a fissile material, a pressure vesselcontaining the nuclear reactor core immersed in primary coolant disposedin the pressure vessel, and an isolation valve assembly including avalve cover having a single open end with a flange, a spool pieceincluding a first flange and a second flange secured with the flange ofthe valve cover to define a sealed volume enclosed by the valve cover, afluid flow line passing through the spool piece and flowing fluid intoor out of the first flange, and a valve supported in the sealed volumeand operatively connected with the fluid flow line.

The reactor can further comprise a reactor coolant inventory andpurification system (RCIPS), wherein the fluid flow line is a makeupline supplying makeup coolant water from the RCIPS to the pressurevessel and the valve is a check valve preventing backflow of coolantwater from the pressure vessel to the RCIPS.

In another embodiment, the reactor can further comprise a reactorcoolant inventory and purification system (RCIPS), wherein the fluidflow line is a letdown line and the valve is an actuated valve that isopened by an actuation signal to initiate flow of coolant water throughthe letdown line from the pressure vessel to the RCIPS.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary nuclear reactor includinga reactor vessel and a reactor coolant inventory and purification system(RCIPS).

FIG. 2 is a perspective view of an exemplary vessel isolation valve inaccordance with the present disclosure.

FIG. 3 is a perspective cutaway view of the isolation valve of FIG. 2.

FIG. 4 is a perspective view of another exemplary isolation valve inaccordance with the present disclosure shown with the isolation valvevessel in phantom to reveal interior components.

FIG. 5 is an enlarged view of a portion of the isolation valve of FIG. 4with the valve vessel removed to expose the interior components of theisolation valve.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of a nuclear reactor including apressure vessel 10. The pressure vessel 10 contains a nuclear reactorcore 11 (shown in phantom) disposed at or near the bottom of thepressure vessel 10 and immersed in primary coolant water also disposedin the pressure vessel 10. The pressure vessel 10 further containsnumerous internal components that are not shown in FIG. 1 but which areknown in the art, such as structures defining a primary coolant flowcircuit, e.g. a hollow cylindrical central riser defining a hot leginside the riser and a cold leg in a downcomer annulus (e.g., flowregion) defined between the central riser and the pressure vessel 10,and neutron-absorbing control rods and associated drive mechanisms forcontrolling reactivity of the nuclear reactor core. Some embodiments,e.g. integral pressurized water reactor (PWR) designs, also include oneor more steam generators disposed inside the pressure vessel, typicallyin the downcomer annulus.

A reactor coolant inventory and purification system (RCIPS) 12 isprovided to maintain the quantity and purity of primary coolant insidethe pressure vessel. A letdown line 14 removes primary coolant waterfrom the pressure vessel 10 into the RCIPS 12, and a makeup line 16delivers makeup primary coolant water from the RCIPS 12 to the pressurevessel 10. The RCIPS 12 includes a pump 17 and other water processingcomponents (not shown) for purifying and storing reserve primarycoolant, injecting optional additives such as a soluble boron compound(a type of neutron poison optionally used to trim the reactivity), or soforth. Isolation valves 20, 21 are provided at respective vesselpenetration locations where the letdown line 14 and makeup line 16,respectively, pass through an outer wall 18 of the pressure vessel 10.During ordinary operation, makeup water flows into, and/or letdown waterflows out of, the pressure vessel 10 through the letdown line 14 andmakeup line 16 to maintain desired operating volume and composition(e.g, purity) of the primary coolant water in the pressure vessel 10.However, if a break occurs in one of the fluid flow lines 14, 16, orelsewhere, such that a LOCA is initiated and uncontrolled primarycoolant water discharge might occur, then flow of coolant out of thepressure vessel 10 is automatically blocked by the affected valve 20,21.

With reference to FIG. 2, an exemplary letdown isolation valve assembly20 includes an isolation valve vessel (IVV) with a small pressureboundary containing redundant isolation valves. The pressure boundary isdesigned to withstand operating pressure and temperature conditions ofprimary coolant inside the pressure vessel 10. The isolation valvevessel is mounted to the side of the lower vessel with a flangedarrangement 32, which in the illustrative example is a spool piece 32.As used herein, a spool piece includes two flanges connected by pipingor another passageway. The spool piece is rated to withstand theoperating pressure of the pressure vessel 10, and in some embodimentsthe spool piece 32 is a forging. One flange of the spool piece 32 isconnected with a mating flange of the pressure vessel 10 to connect theisolation valve assembly 20 directly to the wall 18 of the pressurevessel 10. The other flange of the spool piece 32 is connected with aflanged open end of the isolation valve vessel to define a sealedvolume. Any leakage at the valves is contained within this sealedvolume.

With additional reference to FIG. 3, the details of the exemplaryisolation valve assembly 20 in accordance with the disclosure will bedescribed. The illustrated valve assembly 20 is a letdown isolationvalve that can be used to control the flow of fluid out of the reactorcore. However, it will be appreciated that the valve 20 could also beinstalled on a makeup line for adding fluid to the reactor core, or inanother fluid line feeding into and/or out of the pressure vessel 10.

The valve 20 includes the spool piece 32 and an isolation valve vessel34 secured together via a mating flange 36 at a (single) open end of theisolation valve vessel 34 and a flange 38 of the spool piece 32. Thespool piece 32 also includes a mounting flange 42 having a centrallylocated inlet/outlet 44 and a plurality of bolt holes surrounding theinlet/outlet 44 for securing the valve assembly 20 to a mating flange 48of a pressure vessel, such as pressure vessel 10. Thus, the spool piece32 includes a first flange (namely the mounting flange 42) and a secondflange (namely the flange 38 that connects with the isolation valvevessel 34). The spool piece 32 further includes a passageway 46connecting the first and second flanges 42, 38. In the illustrativeexample, the mounting flange 42 is spaced apart from the flange 38 andconnected by the passageway 46 which is a reduced diameter section. Theisolation valve vessel 34 includes a hemispherical or elliptical head 52(e.g., a valve cover) having flange 36 which connects with the flange 38of the spool piece 32. The connection of the isolation valve vessel 32and the flange 36 defines a sealed volume contained by the isolationvalve vessel 32. A fluid flow line 54 includes a “U”-shaped portiondisposed inside the isolation valve vessel 32 and then continues oncoaxially inside the spool piece 32 to flow fluid into or out of theflange 42. In the illustrative example of letdown valve assembly 20,fluid flows from the pressure vessel 10 through the fluid flow line 54and into the letdown line 14 (see FIG. 1) to reduce the quantity ofprimary coolant in the pressure vessel 10.

When the letdown valve assembly 20 is mounted to pressure vessel 10, theinlet/outlet 44 serves as an inlet that is in fluid communication withthe interior of the pressure vessel 10 such that primary coolant canflow from the pressure vessel 10 through the letdown valve assembly 20via valve fluid line 54 to an inlet/outlet 56 of the valve assembly 20.In the illustrative case of letdown valve assembly 20, the inlet/outlet56 serves as an outlet that is connected to the letdown line 14 of theRCIPS 12. The illustrated “U”-shaped portion of the fluid flow line 54inside the isolation valve vessel 34 advantagely accommodates thermalexpansion.

Isolation valve vessel 34 together with the flange 38 define a sealedinterior volume or chamber C in which a pair of valves 60 and 62 aresupported. (In view of this, the hemispherical or elliptical head 52 isalternatively referred to herein as valve cover 52). In the illustrativeexample of letdown valve assembly 20 which is configured for a letdownapplication, the valves 60 and 62 are suitably actuated valves which areopened (or closed) by an actuation signal. Typically, it is preferableto have the valves 60, 62 be “normally closed” valves such that theactuation signal causes the valves to open so that the valves are closedin the passive state, although a “normally open” configuration is alsocontemplated. In some embodiments the valves 60, 62 are pneumaticallyactuated ball valves, although valves employing electrical, hydraulic,or manual actuation are also contemplated, as are valves other than ballvalves.

In the makeup valve configuration (e.g., the makeup valve assembly 21 ofFIG. 1), the valves 60 and 62 can be swing check valves or another typeof check valve, which is configured to prevent fluid flow into theflange 42 (i.e., configured to prevent flow of primary coolant out ofthe pressure vessel 10). The valves 60 and 62 are arranged in series forredundancy, and it will be appreciated that additional valves, or asingle valve, could be provided in the chamber C as desired. Theisolation valve vessel 32 optionally includes various penetrations forthe plant instrument air system to pressurize the chamber C for vesselleak testing, and for air lines 64 for piloting/actuating the pneumaticactuators in case of pneumatically actuated valves.

An optional internal support structure 68 is secured to flange 38 tosupport the actuated valves 60 and 62 (or to support the check valves inthe case of makeup isolation valve assembly 21). The support structure68 optionally also serves as a mechanical guide for installing the valvecover 52 so that it does not impact any internal components (e.g.,valves and/or actuators, etc.) when it is removed and/or installed toallow maintenance access. Thermal insulation, although not illustrated,can be provided and its location will depend on the design of theactuator and/or position indicators. If high temperature actuators areutilized, the insulation can be placed on the outside of the supportstructure 68 and cover 52. If actuator temperature limitations preventsuch positioning of the insulation, multi-layer metal insulation can beprovided on the piping and a component cooling water line can be addedto actively cool the valve 20 to assure acceptable temperatures. Thesupport structure 68 is optional—in some embodiments the “U” shapedportion of the fluid flow line 54 has sufficient rigidity to support thevalves 60, 62.

In the illustrated embodiment, an optional third isolation valve 70disposed outside of the chamber C is provided to isolate the valve fluidline 54 in the event of a pipe break inside of the isolation valvevessel 34. The external valve 70 can be pneumatically operated, forexample, and configured to close the valve fluid line 54 in the event ofa leak within the valve 20. The third isolation valve 70 can be used,for example, to block flow through the valve fluid line in the event theother valves are disabled due to flooding of the chamber C during aninternal pipe break and/or leakage event. Third isolation valve 70provides a level of redundancy.

Turning to FIGS. 4 and 5, another exemplary isolation valve assembly 100in accordance with the disclosure is illustrated. In this embodiment,the valve assembly 100 is similar to the valve assembly 20 of FIGS. 2and 3. However, the valve assembly 100 has valves supported by the“U”-shaped portion of the fluid flow line (i.e., the support structure68 is omitted), and valve actuators are mounted external to the pressurevessel. To this end, the valve 100 generally includes a spool piece 104and an isolation valve vessel 108 comprising a valve cover 112 includinga flange 116 that is removably secured to a mating flange 120 of thespool piece 104 with bolts or other fasteners (not shown). The valveassembly 100 is mountable to a pressure vessel of a nuclear reactor orother component via a mounting flange 124 of the spool piece 104 that isaxially spaced from flange 120 of the spool piece 104 by a passageway122. A fluid flow line 128 fluidly connects an inlet/outlet (not shown)of the mounting flange 124 with an inlet/outlet 132.

As with valve assembly 20, the valve assembly 100 includes an interiorchamber C formed by the valve cover 112 and the flange 120 secured tothe flange 120 of the spool assembly 104, and a pair of valves 140 and142 are supported inside the chamber C. Valves 140 and 142 are supportedby valve fluid line 128 and are arranged in series for redundantlyblocking flow through the valve fluid line 128.

In embodiment of FIGS. 4 and 5, externally mounted valve actuators 146and 148 are provided for actuating valves 140 and 142. To this end, theactuators 146 and 148 are mounted to respective actuator flanges 152 and154 on the valve cover 112 with bolts or other suitable fasteners (notshown). A connecting shaft 156 (see FIG. 5) extends from the valves 140through the valve cover 112 for coupling with the actuator 146. In oneembodiment having ball valves, rotation of the connecting shaft 156 bythe actuator 146 moves a ball of the valve 140 between respective openand closed positions. Valves 142 includes a similar configuration,although its connecting shaft is not visible in the drawings.

This configuration places the actuators 146, 148 outside of therelatively harsh environment of the chamber C, and therefore canincrease component longevity and/or allow the use of conventionalactuators. This generally simplifies the design and potentiallyeliminates the need for thermal insulation inside the pressure vessel.The connecting shafts for connecting the actuators to the valve memberintroduce the potential for some leakage around the connecting shafts,but leakage up to several gallons per minute or more can be accommodatedwhile still achieving acceptable performance. As an alternativeapproach, a wireless actuation signal is also contemplated, which wouldeliminate the penetrations through the valve cover 112.

The isolation valve vessel of the present disclosure provides isolationfor any pipe break of the makeup or letdown lines, assuming any activecomponent failure. The makeup lines with check valves will automaticallyclose if flow reverses, isolating the LOCA. The letdown lines requireclosure of the ball valves which is effected via the pneumatic actuatorsand occurs on a low RCS pressure signal.

Elimination of the low break LOCA simplifies design basis accidentanalysis and eliminates sump recirculation after a LOCA. The valves inthe vessel would isolate the broken line and long term makeup andletdown would continue using the non-effected lines. Because of thelimited volume of the vessel, the amount of debris that can flow intothe RCS is significantly limited, reducing concerns of debris pluggingof flow passages in the fuel assemblies.

It will now be appreciated that the present disclosure provides at leastone or more of the following advantages:

1. Eliminates the two separate valve rooms used in conventionalreactors.

2. Eliminates the Type 1 LOCA described above. Type 1 LOCA is generallyconsidered the most difficult type of failure in which to provide longterm cooling because most of the water spills on the refueling cavityfloor. The RWST level drops to approximately 8 ft above the lower vesselpenetrations minimizing the driving head to inject water.

3. The higher driving head allows greater flexibility in automaticdepressurization valve sizing because very low differential pressures(e.g., less than 5 psi) are not required for long term injection.

4. During long-term cooling, there is a potential for water to flowthrough the break back into the reactor vessel. The invention limits thewater that can flow back into the vessel and, because it is a closedstructure, limits the amount of fibrous debris that can be mixed withthe water.

5. By eliminating the Type 1 LOCA and its low passive injectionpressure, the ADV and upper vessel penetration sizes may be reduced,making any upper breaks more benign.

6. The vessel reduces the length of ASME Class I piping.

The exemplary embodiment has been described with reference to thepreferred embodiments. Obviously, modifications and alterations willoccur to others upon reading and understanding the preceding detaileddescription. It is intended that the exemplary embodiment be construedas including all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

The invention claimed is:
 1. An apparatus comprising: an isolation valveassembly including: an isolation valve vessel including a single openend; a mounting flange sealing with the isolation valve vessel to definea sealed volume; a fluid flow line in fluid communication with themounting flange to flow fluid through the mounting flange; and a valvedisposed in the isolation valve vessel inside the sealed volume andoperatively connected with the fluid flow line, wherein the fluid flowline both enters and exits the isolation valve vessel through the singleopen end.
 2. The apparatus of claim 1, wherein the isolation valveassembly further includes a forging including the mounting flange and asecond flange to which the isolation valve vessel is secured, theforging having a passageway extending between the mounting flange andthe second flange through which the fluid flow line passes.
 3. Theapparatus of claim 1, wherein the valve is a check valve allowing flowin a first direction but not a second opposite direction within thefluid flow line.
 4. The apparatus of claim 1, wherein the isolationvalve assembly further includes an external isolation valve disposedoutside the isolation valve vessel and outside the sealed volume andoperatively connected with the fluid flow line.
 5. The apparatus ofclaim 1, further comprising: a nuclear reactor comprising (i) a pressurevessel including a mating flange and (ii) a nuclear reactor corecomprising fissile material disposed in the pressure vessel; wherein themounting flange of the isolation valve assembly is connected with themating flange of the pressure vessel of the nuclear reactor.
 6. Theapparatus of claim 5, further wherein the fluid flow line is a makeupline of a reactor coolant inventory and purification system (RCIPS) andthe valve is a check valve preventing backflow of reactor coolant fromthe pressure vessel into the makeup line.
 7. An isolation valve assemblyfor use with a nuclear reactor including a pressure vessel, comprising:an isolation valve vessel having a single open end with a flange; aspool piece having a first flange secured to the pressure vessel, and asecond flange secured to the flange of the isolation valve vessel; afluid flow line passing through the spool piece to conduct fluid flowinto or out of the first flange wherein a portion of the fluid flow lineis disposed in the isolation valve vessel; and at least one valvedisposed in the isolation valve vessel and operatively connected withthe fluid flow line, wherein the fluid flow line both enters and exitsthe isolation valve vessel through the single open end.
 8. The isolationvalve assembly of claim 7, wherein the at least one valve is a checkvalve preventing fluid flow out of the pressure vessel.
 9. The isolationvalve assembly of claim 8, wherein the fluid flow line is a makeup linefor supplying reactor coolant to the pressure vessel and the at leastone valve is a check valve preventing primary coolant from flowing outof the pressure vessel through the fluid flow line.
 10. The isolationvalve assembly of claim 7, wherein an end of the fluid flow line isdisposed coaxially inside the spool piece.
 11. The isolation valveassembly of claim 7, further comprising a redundant valve disposedoutside of the isolation valve vessel and operatively connected with thefluid flow line.